<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>

<script>

    <!-- NaN和任意数字相加都等于NaN -->


  /*  function add(a, b) {
        return a+b;
    }

    let result=add(3,5);
    document.write(result);*/

  var add=function(a,b){
      return a+b;
  }

  let result1=add(3,5);
  document.write(result1);

  let result=add(3,5,7);
  document.write(result);

  let result2=add(3);
  document.write(result2);


</script>
</body>
</html>